Apparatus for biological treatment of waste gases

ABSTRACT

Method and apparatus for removal of malodorous and/or toxic components from wast gases by biological treatment of the same. Such removal is accomplished by passing the gas stream through a humidifying temperature conditioning section (2) and then through a filter bed (3). The filter bed consists of a mixture of an organic biological active material, like compost, an inert fraction and additives for neutralization and organic buffering. Also disclosed is the inoculation of the filter bed with specific micro-organisms adapted to remove constituents of the gas stream which are difficultly biodegradable, e.g. methylenechloride.

This application is a continuation of applications Ser. No. 922,716filed Oct. 24, 1986 now abandoned which application is a continuation ofSer. No. 771,966 filed Sept. 3, 1985 now U.S. Pat. No. 4,662,900; whichapplication are a continuation-in-part of application Ser. No. 645,298filed Oct. 24, 1984 now abandoned.

The invention relates to an apparatus of biologically filtering gases,in particular waste gases, by passing them through a layer of carriermaterial which has been provided with appropriate micro-organisms, andto an apparatus for carrying out the method.

For recent years biological filtration is increasingly used for cleaninggas streams originating from various industrial activities likeproduction processes, waste water transport and waste water treatment inorder to reduce nuisance, trouble or damage to the environment caused bymalodorant and/or poisonous components. In view of the material oftenused for biological filtration, one also speaks of compost filtration.In biological filters other materials like peat, wood chips orfit-branches are used as the so called carrier materials.

In the biological method the removal of harmful components from the gastakes place by activity of micro-organisms, mainly bacteria and fungi,which are present on or in the carrier material. The carrier materialsfor that purpose are disposed in a container. The assembly of container,carrier materials and micro-organisms usually is called a biologicalfilter. The gas to be treated is introduced into the container by a fan,the gas passing through the carrier material from above to below or viceversa. The components to be removed from the gas are adsorbed at thecarrier material and converted by the microorganisms present on or inthe carrier material into substances which are not harmful to theenvironment.

A method and apparatus of the kind mentioned in the preamble aredisclosed in the Dutch patent applications No. 81.04987 and in theGerman patent applications Nos. 2.445.315, 2.558.256 and 2.605.606.

The Dutch patent application No. 81.04987 discloses a biological filterfor cleaning gases, wherein relatively large inactive particles of thestarting compost are added to the active compost with the primary objectof reducing the pressure drop across the filter and hence the energyconsumption.

According to the Westgerman patent application No. 2.445.315 theactivity of the filter is increased by mixing the carrier material withbentonite earth. Thereby heavy metals which adversely affect thebiological activity of micro-organisms are chemically bound to bentoniteso as to maintain the required biological activity.

It is noted, that the carrier material in biological filters usually iscomposed of particles of such size that besides a reasonable adsorbingsurface also an acceptable flow resistance is guaranteed. A too smalladsorbing surface leads to a too large and uneconomical volume of theflter, whereas a too large filter resistance is accompanied by a toolarge energy consumption in passing the gas stream through the filter.

Furthermore it is noted that for optimal operation of the biologicalfilter a water content of 40-60% in respect of the weight of the carriermaterial is necessary. In the known methods and apparatuses humiditycontrol, if any, is realized by spraying water across the upper surfaceof the filter.

It has now appeared that by spraying water onto or into the carriermaterial it is not or not sufficiently possible to realize an adequatehumidity control. In particular the degree of humidity of the mostactive portion of the filter, i.e. the lower side, when the flow throughthe filter is from below to above, cannot sufficiently be controlled bythe known method. The known method, e.g. by spraying water externally orinternally, is only capable of restoring to the desired water content ofthe carrier material, which by calamities have dried too much.

At too low a moisture content of the carrier material the biologicalactivity will strongly diminish or even totally disappear.

Furthermore, shrink cracks develop in the carrier material inconsequence of drying up resulting in short circuiting the gas flow inthe bed so that a considerable portion of the waste gas is withdrawnfrom the contemplated biological treatment. Shrink cracks can be avoidedto a large extent by the choice of a suitable carrier material.

A too high moisture content in the carrier material results in thedevelopment of anaerobic zones. Such zones have an increased flowresistance precluding a homogeneous flow of gas, which results in aconsiderable decrease of the average residence time of the gas in thefilter bed.

Furthermore the anaerobic zones produce volatile metabolic productsleaving the bed with the waste gas. Since the produced metabolicproducts have malodorant properties, they contribute so the undesiredmalodorant nuisance.

It is noted that the micro-organisms responsible for decomposing themainly organic components which are present in the gas to be treated,require various inorganic nutrients for their metabolism. As basicmaterial various kinds of compost are satisfactory for this purpose.These materials, however, have the common disadvantage of being stronglysubject to aging. Some kinds of compost have a tendency of lumpingaccompanied by a strong decrease of the specific surface area (surfacearea per unit of volume) aging phenomena of the carrier material can becaused by local loss of moisture by a too low relative humidity of theair and/or the occurrence of temperature gradients in the carriermaterial.

The occurrence of temperature gradients in the carrier material isinherent to the microbiological activity.

This activity may be locally different. Where this activity is high, thetemperature will be slightly higher than elsewhere as a result of therealised energy of oxidation.

As a result of these temperature gradients will develop also gradientsin the maximum water vapour pressure. This implies that biologicallyactive zones have a tendency of drying up while less active zones takeup this excess water by condensation.

As a result active zones will experience a loss of activity and wet,mostly inactive zones will take up still more water.

Aging phenomena in the carrier material mostly manifest themselves bythe development of shrink cracks in the carrier material in which dryingoccurs, and by the development of wet, often anaerobic zones. Such agingphenomena are mostly irreversible, which means, that a shrink crack oncedeveloped will not disappear automatically.

It has been known that aging phenomena of this kind can be prevented bycontinuously moving the carrier material throughout the bed and mixingit up outside of the bed as disclosed by the German patent applicationNo. 2.445.315.

Another problem presented by the known methods and apparatuses is thatin the microbiological conversion of certain organic components in thewaste gases acid components, a.o. acetates, are produced as intermediateproducts. Such acid components contribute to acidification of thecarrier material and as a result the microbiological activity willdrastically decrease or even totally disappear.

Furthermore the decomposition of other organic compoments byacid-intolerant organisms can be considerably hampered. This, forexample, is the case with the decomposition of toluene.

In the use of biological filters for cleaning waste gas strongly varyingloadings often occur due to discontinuous processes.

The active micro-organisms in the filter consequently are subject toloadings fluctuating in time. In such cases the concentrations ofpolluting components in the effluent gas may strongly vary unless oneproceeds to the use of a strongly oversized filterbed.

It can be generally stated that these fluctuations may adversely affectmaintaining an optimal microbiological activity. In discontinuous gasdischarges no loading at all takes place at the filter during certainperiods of the day. Though the micro-organisms may well survive duringperiods of shut-down, also of longer duration, for example, weeks, itstill means, that the decomposing capacity of the micro-organisms is notfully exploited.

The invention now has the object of providing a method and apparatus forbiologically filtering gases wherein the above mentioned disadvantagesare removed in an efficient manner.

For this purpose the method of the invention is characterized in thatthe gases prior to being passed through the carrier material are broughtinto intimate contact with water in such a manner that the gases containthe quantity of water required for the optimum activity of themicro-organisms, the gases are brought to the temperature required forthat purpose, and any water soluble substances present in the gasesdissolve at least partially.

The gases to be treated are brought into intimate contact with water byspraying. This brings the gas streams to the right humidity and to therequired temperature as is necessary for the micro-organisms of thecarrier material in the filter to function optimally.

Furthermore as a result of spraying the gases to be treated in asocalled pretreatment chamber the water soluble components of the gasstream will at least partially dissolve in the water so as to be removedfrom the gas stream. Thereby the components which are toxic for themicro-organisms can be removed from the gas stream during thispretreatment so as not to affect the activity of the micro-organisms.

In case of non-toxic but water soluble components decomposable bymicro-organisms a considerable portion thereof can be removed from thegas stream by dissolving. For decomposing the so dissolved components inaccordance with the invention now a specific biological population, forexample, active sledge, can advantageously be introduced into the spraywater.

In this manner a lager quantity of components can be removed per unit oftime from the gases to be treated so as to increase the efficiency ofthe biological filter which subsequently is contacted by the gas.

Research at the Eindhoven University of Technology on which theinvention is based, has shown that adding certain additional materialsto the carrier material yields very favourable results.

The additional materials of the invention can be sub-divided intomaterials preventing the development of shrink cracks in the carriermaterial and reducing the flow resistance of the biological filter onthe one hand, and additional materials counteracting acidification ofthe filtermaterial on the other hand.

In particular it has been shown in accordance with the invention thatmodifying the structure of the carrier material by adding additionalmaterial of the first type may prevent the very disadvantageous agingphenomena. As a result of adding the additional material, whichcurrently is inert material to the carrier material, for example,compost, and by mixing it therewith the development of shrink cracks canbe counteracted.

As additional material of the first-mentioned type, which mainly isinert, and has a diametrical particle size of 3-10 mm, the followingsubstances having a more or less rigid structure can successfully usedin accordance with the invention: organic materials, like polyethylene,polystyrene, particles of ground automobile tires, as well as inorganicmaterials like fired clay particles, ground lava bits, ground coalcinderparticles and pelletized flue gas particles, perlite and active coal.

The proportion of mixing the additional material with the carriermaterial is between 30-70% and 70-30% on the basis of volume.

The additional material should previously be thoroughly mixed with thecarrier material, for example compost, so as to obtain a loose structurehaving a large specific surface.

This mixing can take place simultaneously with other additions ifnecessary or preferable. In this respect, for example, micro-organismswhich by nature are not present in or on the carrier material can beapplied to the carrier material by inoculation.

Furthermore, the research at the Eindhoven University of Technology hasshown that by the choice of a suitable additional material also the flowresistance can be considerably decreased. So, for example, the flowresistance at a surface loading of 200 m³ /m² /hour is about 8 mm headof water per meter of bed height when using a mixture of 50%-60%polyethylene particles (low density nr. 1500, particle diameter about 4mm) and 50%-40% compost.

In contrast therewith there is at a same value of surface loading apressure drop of about 120 mm head of water per meter of bed height whenno polyethylene particles are added.

Porous additional materials having a high internal porosity andhydrophilic properties are advantageous, since such materials mayfunction as a buffer for excess moisture in the carrier material, whichexcess moisture in case of excessive loss of moisture can be yielded bysuch materials. This effects a certain degree of humidity control in thecarrier material.

The disadvantage of acidification of the carrier material can beadvantageously removed by the use of additional materials of the secondtype counteracting the acidification of the filter material. For thispurpose a pH-reducing alkaline substance is added to the carriermaterial and intimately mixed therewith. Advantageously, for example,marl, limestone and calcium carbonate in a weight proportion of 2-40% inrespect of the carrier material can be considered for preventingacidification of the carrier material. It has appeared that inparticular cases (for example, the micro-biological decomposition ofmethylene chloride) considerably higher weight proportions should beapplied, which may range up to a value of 40% in respect of the totalfilling quantity.

The research at the Eindhoven University of Technology besides thepreviously mentioned two types of additional material has shown activecarbon to be very suitable as additional material.

Active carbon added to the carrier material in a quantity of 1-50 kg/m³appears to be very favourable in case the present biological filters ofthe invention are subjected to various loads as in various and/ordiscontinuous processes. The active carbon in such case is or is notmixed with the carrier material. These carbon particles have adsorbingproperties for organic components which are present in the waste gas tobe treated.

Besides active carbon other substances having adsorbing properties maybe used, for example polyethylene glycol.

Besides their good adsorbing properties such substances should also havegood desorbing properties. Thereby it is possible that during a periodof increased supply the adsorbens temporarily adsorbs the concentrationexcess and desorbs it at lower loads on behalf of the micro-biologicaldecomposition. This permits in principle a considerable reduction in thefilter volume required at discontinuously discharging plants.

When very concentrated waste gas streams are discharged during a shortperiod, in order to reduce the volume of the filter a quantity ofadsorbens (active carbon) is required which will be in excess of 50kg/m³ and will be at maximum±250 kg/m³.

At such high concentrations of carbon these particles can also take overthe function of inert carrier material.

It is noted that in accordance with the present method the temperatureof the spray water is chosen so that the temperature of the gases to betreated is between 10° and 40° C.

In accordance with the invention the relative humidity of the gasesafter pretreatment with water is usually 95-100%.

The invention also relates to an apparatus for carrying out the methodof the invention for biologically filtering gases, in particular wastegases.

Such apparatus comprises a container including a biological filter unit,a supply conduit for the gases to be treated at the lower side and adischarge conduit for the gases to be treated at the upper side.

The apparatus of the invention is characterized in that at least onefilter unit is arranged in the container and a pretreatment chamber forthe gases to be treated is disposed under or at the lower side of thelowermost filter unit.

Usually the container is cylindrical, though it may be rectangular or bedifferently shaped.

In the apparatus of the invention the pretreatment chamber and thelowermost filter unit are separated from each other by a support layerpermeable for the gas to be treated. The support layer usually is aperforated metal or plastics plate. The pretreatment chamber is providedwith a spraying water facility for intimately contacting the waste gaseswith water.

In a favourable embodiment the pretreatment chamber is provided with acontact bed arranged above the supply conduit for the gases to betreated and resting on the gas-permeable support layer, a dischargeconduit for spraywater under the support layer and the spraying facilityfor the water above the contact bed. The contact bed enhances theintimate contact between the gases to be treated and the spray water.The contact bed advantageously can be a gravel bed, though othersuitable materials also can be used.

To reduce the quantity of water used, it is advantageous, when thespraying facilities are connected to a discharge conduit for spraywatervia a circulation conduit including a circulation pump for circulatingthe spraywater.

For adjusting the temperature of the spraywater it is favourable whenthe circulation conduit includes a heat exchanger. Adjacent to thepretreatment chamber is a filter unit, which, in a simple embodiment,consists of a gas-permeable support plate and a biologically activezone. Currently, the gas-permeable plate is a perforated metal orplastics plate.

In order to obtain a more homogeneous distribution of the gases to betreated across the biologically active zone use can be made of a contactmaterial on which the biologically active zone rests. As a contactmaterial advantageously lava bits or gravel are used, though othersuitable materials also can be considered for this purpose.

According to the invention the biologically active zone can be composedof a suitable carrier material, for example compost, peat, wood chips,etc. Preferably, the biological active zone is composed of such carriermaterial and an appropriate additional material as discussed previously.

Advantageously, spraying facilities are dispassed in the upper portionof the filter unit in order to prevent the biologically active zone fromdrying up in case of calamities.

There can be circumstances requiring that the apparatus has a pluralityof filter units disposed above each other and separated from each otherby a gas-permeable support plate being a perforated metal or plasticsplate as mentioned previously.

A favourable embodiment of the apparatus is illustrated in the singledrawing.

The container 1 which in this embodiment is cylindrical is composed of apretreatment chamber 2 for the waste gases and a filter unit 3. In thepretreatment chamber is provided the supply conduit for the gases to betreated 4 and the support layer 9 which in this embodiment is aperforated metal plate. The pretreatment chamber may be arrangedseparately from the container.

On the support layer 9 a gravel bed is provided for enhancing thecontact between the upward gas-stream and the downwardly sprayed liquid.

Up in the pretreatment chamber 2 spraying means 5 are provided. Down inthe pretreatment chamber 2 the spray water may be tapped through thedischarge conduit 20 and the valve 21 or it may be circulated throughthe circulation conduit 19 including the circulation pump 6 and the heatexchanger 7. In the heat exchanger 7 the spray water can be brought tothe desired temperature, i.e. when a higher temperature is desired, thespray water can be heated in the heat exchanger or when a lowertemperature is desired, the spray water can be cooled.

Above the pretreatment chamber 2 is provided the filter unit 3 separatedby the gas permeable support plate 10 on which rests the biologicallyactive zone 11. The biologically active zone 11 can be composed of thecarrier material alone but preferably of a mixture of carrier materialand the previously discussed additional materials.

Above the biologically active zone 11 are provided sprayers 12 which incase of emergency, for example, when the spraying means 5 in thepretreatment chamber 2 should fail, can secure the moistening of thebiologically active zone.

On the filter unit 3 is fastened the top section 14 through thefastening means 13, for example a screw means.

The biologically filtered gas can now be discharged to the atmospherethrough the discharge 15 and the valve 16 or it can be conducted throughthe conduit 17 and the valve 18 to a measuring and sampling apparatus.

On the filter unit 3, if desired, a plurality of filter units can bearranged which then can be fastened together by a similar fasteningmeans as 13, for example.

For convenience, such fastening means can be a screw means.

The use of more than one filter unit can be necessary in cases whereinthe waste gases to be treated contain components requiring for theirdecomposition different conditions possibly including differentmicro-organisms or when the waste gases to be treated include onecertain component in such a high concentration that the capacity of onefilter unit is inadequate for sufficiently decomposing it.

By a uniform construction of the filter units the apparatus of theinvention, if desired, can be adapted to biologically cleaning variouswaste gases of different compositions by simple fastening the saidfilter units on each other.

The principle of multiple filter units can also be applied by dividing agas stream to be treated into two of more equal streams and byconducting the separate streams to separate filter units disposed aboveeach other in one column.

When the method is effected in such an apparatus, a considerable furtherreduction of the pressure drop of the gas stream through the filter canbe reached. This decrease of the pressure drop for two units arrangedinparallel theoretically amounts to a factor 4 in comparison with thecase of two units in series: namely a factor 2 for the reduction of thegas loading per m² of filterarea traversed and a factor 2 for thereduction of the filter height.

The apparatus of the invention will now be explained by way of thefollowing examples.

EXAMPLE I

An apparatus according to the invention and including a pretreatmentchamber and five filter units arranged in series was continuously fedwith a synthetic waste gas from a paint spraying shop. The diameter of afilter unit was 15 cm, the filing height was 60 cm, the volume ofcarrier material thus being about 11 1 per filter unit. The compositionof the filter material was 29% by weight of peat compost, 59% by weightof polyethylene particles (low density nr. 1500, average particlediameter about 4 mm), 2% by weight of marl.

The filter material was inoculated with an active sludge suspension anda toluene decomposing organism which was isolated from a ground sampleof a petrol filling station.

The synthetic waste gas contained a mixture of the componentsaethyl-acetate, butyl-acetate, butanol and toluene. The gas outputpractised was 67 l/min corresponding to a gas load of 220 m³ /m² /hour,the temperature was about 20° C. The measured total pressure drop acrossthe filter units at the said gas output was 26 mm head of water.

Under stationary conditions the following concentrations were measured:

    ______________________________________                                        Gas concentration in mg/m.sup.3                                                                    butylace-                                                                              aethylace-                                      Component    toluene tate     tate    butanol                                 ______________________________________                                        supply pretreatment                                                                        958     218      205     127                                     chamber                                                                       discharge pretreat-                                                                        941     214      200     0                                       ment chamber                                                                  discharge first filter                                                                     936     89       95      0                                       unit                                                                          discharge second filter                                                                    928     17       38      0                                       unit                                                                          discharge third filter                                                                     908      0        0      0                                       unit                                                                          discharge fourth filter                                                                    872      0        0      0                                       unit                                                                          discharge fifth filter                                                                     810      0        0      0                                       unit                                                                          ______________________________________                                    

EXAMPLE II

A heavily loaded apparatus including a pretreatment chamber and aone-stage filter unit was fed with a synthetic waste gas which containedaethyl-acetate as a polluting component. The filter unit had a diameterof 15 cm and a filling height of 100 cm, thus a filling volume of about18 l. The filling material of the filter unit consisted of 7920 g ofpeat compost, 108 g of polystyrene particles (diameter about 3 mm) and216 g of marl.

The said filling material was inoculated with micro-organisms from anactive sludge suspension taken from a waste water treatment plant.

The filter unit was daily intermittently loaded: during 8 hours per 24hrs a waste gas at a volume output of 100 l/min and therefore a gas loadof 340 m³ /m² /hour, and having an aethyl-acetate inlet concentration of825 mg/m³ was fed. During the remaining 16 hours aeration of the filterwith clean air (air output 10 l/min) took place.

Eight days after start-up of the filter the dynamic behaviour wasstudied. For that purpose the concentration of aethyl-acetate in thewaste gas from the filter unit was measured after application of theload.

This concentration appeared to increase substantially immediately fromzero value until after about 30 min a stationary waste gas concentrationof about 320 mg/m³ was reached. This concentration was maintained duringthe remaining period of 71/2 hours for which the filter was loaded.

EXAMPLE III

To the filling material in the apparatus described in Example II 549 ofactive carbon was added to be mixed therewith.

Also in this case the filter was daily intermittently loaded inaccordance with the data in Example II, it being understood that at thesame time the inlet concentration of aethyl-acetate in the syntheticwaste gas to be cleaned had been further increased to about 1610 mg/m³.

After the filter in this way had been loaded for about 14 days, thedynamic behaviour was again studied. After application of the load theconcentration of aethyl-acetate in the waste gas during the first 30 minappeared to be substantially zero to slowly increase subsequently.

This increase continued during the remaining time of the loaded period.At the end of this period, after 8 hours, the ethylene-acetateconcentration in the waste gas was about 700 mg/m³.

EXAMPLE IV

An apparatus according to the invention including a pretreatment chamberand three filter units arranged in series was discontinuously fed with awaste gas from a pharmaceutical plant. The diameter of a filter unit was150 cm, the filling height was 100 cm, the volume of carrier materialthus being 1,77 m³ per filter unit. The composition of the fillingmaterial in the filter unit was 21% by weight of peat compost, 39% byweight of marl and 40% by weight of active carbon.

Once a week a waste gas was treated by the filter, said gas a.o.containing a methylene chloride concentration of 500 mg/m³ after thepretreatment chamber. The gas output practised was 350 m³ /hourcorresponding to a gas load of 200 m³ /m² /hour. The discharge time ofthe waste gas was 5 hours. In these 5 hours the methylene chloride wasquantitavely captured in the upper filter unit by adsorption at theactive carbon present in the filling material. After the discharge ofthe waste gas the methylene chloride was desorbed from the active carbonby passing an output of 50 m³ /hour of clean room air through thefilter. The desorbed methylene chloride was adsorbed by the biologicallyactive fraction of the filling material and subsequently biologicallydecomposed.

The biological decomposing capacity of the filling material for therecalcitrant component methylene chloride was obtained in that beforethe start-up a suspension of methylene chloride decomposingmicro-organisms, which had been especially prepared for this purpose,was mixed through the filling material of the upper filter section.

The biological decomposing capacity of methylene chloride at 20° C. was30 g/m³ /hour.

I claim:
 1. Apparatus for biologically filtering gases containingpollutants, in particular industrial waste gases by a fixed bed typefilter material containing a carrier material which has been providedwith appropriate micro-organisms which are stationary within the fixedbed on the surface of the carrier material, wherein the gases areinitially water saturated prior to their entrance into the filtermaterial by bringing the gases into intimate contact with water in sucha manner that the gases contain the quantity of water required for themicro-organism to optimally function the water saturated gases are thendirected into the filter material and passed through it, whereby thepollutants in the water saturated gas come in direct contact with themicro-organism on the surface of the carrier material, the apparatuscomprising a container including at least a biological fixed bed typefilter unit including the fixed bed having a carrier material, a supplyconduit for the gases to be treated at the lower side and a dischargefor the treated gases at the upper side, characterized in that at leastone filter unit is arranged in the container, means for moisturizingwith water the gases prior to entry of the gases into the fixed bedincluding a pretreatment chamber, means for introducing gases to betreated into the pretreatment chamber, means for controlling thetemperature of the water for moisturizing the gases, and the fixed bedlocated downstream of the moisturizing means, means for directing thepretreated gases to the filter unit.
 2. Apparatus of claim 1,characterized in that the moisturizing means includes a pretreatmentchamber arranged separately from the container.
 3. Apparatus of claim 1,characterized in that the container is cylindrical.
 4. Apparatus ofclaim 1, characterized in that the moisturizing means includes apretreatment chamber and the lowermost filter unit separated from eachother by a support layer permeable for the gas to be treated. 5.Apparatus of claim 4, characterized in that the gas-permeable supportlayer is a perforated metal or plastics-plate.
 6. Apparatus of claim 1,characterized in that the moisturizing means includes a pretreatmentchamber provided with spraying facilities for the water.
 7. Apparatus ofclaim 1, characterized in that the moisturizing means includes apretreatment chamber provided with a contact bed arranged above thesupply conduit for the gases to be treated and resting on thegas-permeable support layer, a discharge conduit for spraywater underthe support layer and the spraying facilities for the water above thecontact bed.
 8. Apparatus of claim 7, characterized in that the contactbed is a gravel bed.
 9. Apparatus of claim 7, characterized in that thespraying means are connected to a discharge conduit for spraywater via acirculation conduit including a circulation pump for circulating thespraywater.
 10. Apparatus of claim 9, characterized in that thecirculation conduit includes a heat exchanger.
 11. Apparatus of claim 1,characterized in that the filter unit comprises a gas-permeable supportplate and a biologically active zone.
 12. Apparatus of claim 11,characterized in that the gas-permeable support plate is a perforatedmetal or plastics-plate.
 13. Apparatus of claim 11, characterized inthat the biologically active zone rests on a contact material. 14.Apparatus of claim 13, characterized in that said contact material isselected from lava bits, and gravel.
 15. Apparatus of claim 11,characterized in that the biologically active zone is composed of asuitable carrier material and an appropriate additional material asdefined in the preceding method claims.
 16. Apparatus of claim 1,characterized in that spraying facilities are disposed in the upperportion of the filter unit.
 17. Apparatus of claim 1, characterized inthat in case of more than one filter unit these are separated by agas-permeable support plate.
 18. Apparatus of claim 17, characterized inthat the support plate is a perforated metal or plastics-plate.